The present invention is directed generally to solid state radiation detectors, and more particularly to solid state detectors for the detection of submillimeter, millimeter, or infrared wave radiation.
There is increasing interest in the use of millimeter and submillimeter waves for military detection purposes. Wavelengths in the millimeter and submillimeter range have the advantage that they can penetrate smoke and clouds, while infrared waves cannot. Likewise, radiation in this wavelength region provides good spatial resolution, while requiring only a small antenna. These resolution and antenna features are in direct contrast to the requirements for microwave and radar wave devices. Additionally, all of the circuit components for millimeter and submillimeter wave devices are smaller than microwave and radar device components.
One of the major problems in fabricating a detector for millimeter and submillimeter waves is that the energy of the photon in these wavelengths is proportional to frequency. As the frequency of the radiation decreases from the infrared region to the submillimeter region to the millimeter region, the energy of the light photons decreases. For example, for the infrared wavelength of 10 microns, the energy per photon is 120 meV. Note that this infrared wavelength is in the 8-12 micron window. In contrast, for a wavelength of 1 millimeter (1000 microns), the energy per photon is 1.2 meV. This is a reduction in energy by two orders of magnitude from the energy of the infrared photon. Thus a material must be found with bound charges which can be excited by very small photon energies of on the order of 1.2 meV so that millimeter waves impinging on such a device would be sufficient to remove these bound charges to thereby increase the conductivity of the device. This change in conductivity could then be measured as an indication of the reception of the millimeter wave.
It is known that some doped semiconductors will form D.sup.- and A.sup.+ centers under certain circumstances. In this regard, neutral impurity doping atoms can attract an additional charge carrier through the mechanism of sharing the impurity atoms's charge with this extra charge carrier. A D.sup.- center is formed when a neutral impurity donor added to a semiconductor binds not only the electron that it would normally bind, but also weakly binds a second electron thereto. In the case of the donor atom, this second extra electron is bound via the sharing of the positive charge at the atom's nucleus with the second electron. The energy binding this second electron to the neutral impurity donor is small enough that when a photon from a millimeter or submillimeter wave impinges on the D.sup.- center, this second weakly held electron is excited into the conduction band. Likewise, when a neutral acceptor impurity is added to a semiconductor, then the neutral acceptor atom will bind its own hole, and may also trap an extra hole very weakly. Accordingly, this A.sup.+ center with its trapped extra hole has a positive charge. Again, the very small energy obtained from the photon of a submillimeter or millimeter wave will be sufficient to excite this second trapped hole into the conduction band for the material.
The basic problem for this type of device is that the number of steady state D.sup.- centers formed in a donor-doped semiconductor is very low. Likewise, the number of A.sup.+ centers formed in an acceptor-doped semiconductor is also very low. In order to increase the D.sup.- centers or A.sup.+ centers so that when a millimeter or submillimeter wave impinges on the material, a sufficient number of carriers will be excited into the material's conduction band so that a measurable response can be detected, an optical bias is required. Typically, the doped semiconductor device is flooded with an infrared optical bias beam. This infrared optical bias beam causes substantial photoconductivity, i.e., many electrons are excited into the material's conduction band. These ionized electrons then recombine with the various impurity centers in the semiconductors material. Statistically, a certain percentage of the carriers will combine weakly with neutral impurity atoms, resulting in charged impurity atoms. In the case of a donor impurity, a certain percentage of the excited electrons will combine weakly with the neutral donor impurity atom to form a D.sup.- center.
The optical bias required to establish an appreciable steady state of D.sup.- or A.sup.+ centers results in a rather large, optically induced dark current in the device. This large dark current caused by the optical bias, in turn, causes shot noise in the device which limits the performance of the device to infrared detection applications. This shot noise is the statistical noise associated with the charge carriers moving from across one electrode to another in the device. The shot noise is proportional to the square root of the dark current resulting from the optical bias and effectively acts to prevent accurate detection of small conductivity changes in the device.
Accordingly, the problem confronting the art is how to minimize the dark current in the semiconductor device to thereby minimize the shot noise. In essence, the problem is to form the D.sup.- and A.sup.+ centers without flooding the semiconductor material with optical bias light which causes the resulting dark current.